Conventional ultrasound scanners create two-dimensional B-mode images of tissue in which the brightness of a pixel is based on the intensity of the echo return. Alternatively, in a color Doppler mode, the movement of fluid (e.g., blood) or tissue can be imaged. Measurement of blood flow in the heart and vessels using the Doppler effect is well known. The phase shift of backscattered ultrasound waves may be used to measure the velocity of the backscatterers from tissue or blood. The Doppler shift may be displayed using different colors to represent speed and direction of flow. Alternatively, in power Doppler imaging, the power contained in the returned Doppler signal is displayed.
Conventional ultrasound imaging systems comprise an array of ultrasonic transducer elements arranged in one or more rows and driven with separate voltages. By selecting the time delay (or phase) and amplitude of the applied voltages, the individual transducer elements in a given row can be controlled to produce ultrasonic waves which combine to form a net ultrasonic wave that travels along a preferred vector direction and is focused at a selected point along the beam. The beamforming parameters of each of the firings may be varied to provide a change in maximum focus or otherwise change the content of the received data for each firing, e.g., by transmitting successive beams along the same scan line with the focal point of each beam being shifted relative to the focal point of the previous beam. In the case of a steered array, by changing the time delays and amplitudes of the applied voltages, the beam with its focal point can be moved in a plane to scan the object. In the case of a linear array, a focused beam directed normal to the array is scanned across the object by translating the aperture across the array from one firing to the next.
The same principles apply when the transducer probe is employed to receive the reflected sound in a receive mode. The voltages produced at the receiving transducer elements are summed so that the net signal is indicative of the ultrasound reflected from a single focal point in the object. As with the transmission mode, this focused reception of the ultrasonic energy is achieved by imparting separate time delay (and/or phase shifts) and gains to the signal from each receiving transducer element.
A single scan line (or small localized group of scan lines) is acquired by transmitting focused ultrasound energy at a point, and then receiving the reflected energy over time. The focused transmit energy is referred to as a transmit beam. During the time after transmit, one or more receive beamformers coherently sum the energy received by each channel, with dynamically changing phase rotation or delays, to produce peak sensitivity along the desired scan lines at ranges proportional to the elapsed time. The resulting focused sensitivity pattern is referred to as a receive beam. A scan line's resolution is a result of the directivity of the associated transmit and receive beam pair.
A B-mode ultrasound image is composed of multiple image scan lines. The brightness of a pixel is based on the intensity of the echo return from the biological tissue being scanned. The outputs of the receive beamformer channels are coherently summed to form a respective pixel intensity value for each sample volume in the object region or volume of interest. These pixel intensity values are log-compressed, scan-converted and then displayed as a B-mode image of the anatomy being scanned.
In addition, ultrasonic scanners for detecting blood flow based on the Doppler effect are well known. Such systems operate by actuating an ultrasonic transducer array to transmit ultrasonic waves into the object and receiving ultrasonic echoes backscattered from the object. In the measurement of blood flow characteristics, returning ultrasonic waves are compared to a frequency reference to determine the frequency shift imparted to the returning waves by flowing scatterers such as blood cells. This frequency, i.e., phase, shift translates into the velocity of the blood flow. The blood velocity is calculated by measuring the phase shift from firing to firing at a specific range gate.
The change or shift in backscattered frequency increases when blood flows toward the transducer and decreases when blood flows away from the transducer. Color flow images are produced by superimposing a color image of the velocity of moving material, such as blood, over a black and white anatomical B-mode image. Typically, color flow mode displays hundreds of adjacent sample volumes simultaneously laid over a B-mode image, each sample volume being color-coded to represent velocity of the moving material inside that sample volume at the time of interrogation.
Ultrasound scanners which perform color Doppler imaging employ an ROI which specifies the area of the gray-scale B-mode image to overlay with color Doppler data. The ROI is often made smaller than the B-mode image in order to maintain an acceptable acoustic frame rate. The scanner is programmed to allow the operator to move the ROI about the B-mode image area. In the case where a straight linear transducer is used, both the B-mode image area and the ROI are rectangles. Thus, as the depth of the ROI is changed, there is no need to automatically change the height or width of the ROI. However, in the cases where either a curved linear or a sector transducer is used, the scanner is programmed to automatically adjust the ROI size as the operator moves the ROI about the B-mode image area. In accordance with the conventional algorithm, the ROI is typically placed on or near the center of the B-mode image area. If the operator moves the ROI deeper in the image, the height of the ROI remains unchanged and the width of the ROI is changed automatically to accommodate the same number of vectors that were contained in the ROI at its previous position. Since the vectors are diverging with depth, the ROI width is increased as its depth increases. If instead the operator moves the ROI shallower in the image, the same algorithm is used, which results in a narrower ROI. Following the change in ROI position initiated by the operator and the automatic change in ROI width in response to that position change, the operator may then adjust the ROI width to restore the original ROI width. This latter adjustment is desirable in the case where the depth of the ROI is increased because the resulting acoustic frame rate will be increased. This conventional method of operating an ultrasound scanner has the disadvantage that an additional adjustment must be made by the operator following increase in ROI depth in order to gain the benefit of increased acoustic frame rate.